Negative thermal expansion system (NTEs) device for TCE compensation in elastomer compsites and conductive elastomer interconnects in microelectronic packaging

ABSTRACT

A method for fabricating a negative thermal expanding system device includes coating a wafer with a thermally decomposable polymer, patterning the decomposable polymer into repeating disk patterns, releasing the decomposable polymer from the wafer and forming a sheet of repeating patterned disks, suspending the sheet into a first solution with seeding compounds for electroless decomposition, removing the sheet from the first solution, suspending the sheet into a second solution to electrolessly deposit a first layer material onto the sheet, removing the sheet from the second solution, suspending the sheet into a third solution to deposit a second layer of material having a lower TCE value than the first layer of material, separating the patterned disks from one another, and annealing thermally the patterned disks to decompose the decomposable polymer and creating a cavity in place of the decomposable polymer.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Divisional Application of U.S. application Ser. No. 11/932,385filed on Oct. 31, 2007 now U.S. Pat. No. 7,556,979, which is aDivisional Application of Ser. No. 10/310,532, now U.S. Pat. No.7,417,315 filed on Dec. 5, 2002, the disclosures of which are hereinincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention, generally, relates to MicroelectromechanicalSystems (MEMs) processes commonly used in semiconductor manufacturing,but applied to composite materials and “smart materials” or “responsivematerials”. More particularly, the present invention relates to methodsfor incorporating a negative thermal expansion system (NTEs) device inelastomer or soft composite materials and in conductive elastomerinterconnects in microelectronic packaging.

2. Description of Related Art

In many areas of technology, the difference in coefficient of thermalexpansion (TCE) between bonded parts layers creates stresses that arehighly problematic. In many cases such stresses are limiting factorsbecause the strength of the materials or the interfaces between them areunable to withstand them during temperature excursion. When thematerials in question have high elastic modulae, the TCE mismatchproblem is exacerbated. When they are softer, the mismatch is partlymitigated by elastic deformation. However, this does not fully counterthe problems associated with TCE mismatch, and indeed there is a classof uses for elastomers in technology for which the considerations arequite different. These are when the elastomer is employed to provide arestoring force while in compression. When such is the case, therestoring force will be reduced upon a decrease in ab-21solutetemperature due to the TCE-driven contraction. Indeed, if the restoringforce is small and the temperature decrease large, the elastomer cantransition from being in compression to being in tension. This assumesan adhesive bond. If no such adhesive bond exists, or if the adhesionfails during the compression/tension transition, then contact may belost altogether when the restoring force becomes less than zero. If therole of the elastomer is both to provide adequate restoring force and toprovide conductivity (either electrical or thermal) then thatconductivity will suddenly be interrupted upon loss of contact.

To minimize this problem materials have traditionally been engineered ina variety of ways to have a low TCE while balancing other necessaryproperties. One such approach has been to form composites with a low TCEmaterial in a host polymer matrix. Typically, quartz (SiO2) filler in athermoset polymer like epoxy. In another example, the organic fiberKevlar is known to have a negative TCE in the fiber direction (only) andcomposites made with oriented Kevlar strands have reduced TCE in thatdirection. Many low or negative TCE materials have drawbacks, which havemade them unattractive for some applications, notably microelectronics.

In addition, the control of thermal expansion is particularly importantin elastomers (e.g. rubber), which has a notoriously high expansioncoefficient limiting its use in many high technology applications. Ofparticular immediate interest is the fabrication of small conductingelastomer interconnect contacts for high-end microelectronic packaging.In traditional examples of such contacts, an electrically conductingmaterial such as metallic silver particles are mixed into siloxanerubber and molded into small conducting contacts. These contacts arefabricated into a 2-dimensional array and used as a so-called Land GridArray (LGA) connection between a chip module and a printed circuitboard. However, because these contacts have a high TCE, they areunreliable and are rendered unsuitable for use in high performancecomputers where reliability on an individual contact basis must bemeasured in failure rates at the ppm to ppb level. This high reliabilityrequirement stems from a full system dependence on non-redundant signalcontacts—if even one out of many thousands fail, an entire node or theentire computer can fail. If the TCE could be reduced in such typicalcontacts while maintaining the desirable properties such as elasticityand conductivity, this would significantly increase the reliability.This in turn would reduce the cost of replacing chip modules in thefield by allowing field replacibility of chip modules using LGAinterconnects.

Herein we discuss an innovative approach based on the fabrication of amultitude of negative thermal expanding systems devices (NTEs) that havesignificantly negative coefficients of thermal expansion, and on theincorporation of such NTEs into an elastomer to form a composite withreduced, zero, or negative net TCE. This approach addresses a number ofgeneral engineering concerns such as the reduction of TCE-based stressesto levels that allows fabrication of structures not previously possibleand such as extending the operating conditions under which elastomercomposites will be able to maintain positive restoring forces toopposing surfaces. In particular we disclose herein the fabrication ofLGA interconnect devices using such composite materials as theconducting elastomer.

These NTEs devices may also be used to form negative thermal expansionfoams by fusing or adhering the NTEs together with no host elastomer.

The general concept of negative TCE micro machines is disclosed, as areprocess techniques and composite structures. Also disclosed arepreviously unidentified applications for negative TCE composites ingeneral.

SUMMARY OF THE INVENTION

This disclosure teaches the fabrication of mechanical devices that havea negative coefficient of thermal expansion (TCE) and optionally theirinclusion as filler in another material, such as a soft rubber material.The anticipated size scale is primarily in the micrometer range, thoughboth larger and smaller are also anticipated. It also describes howthese NTEs can be incorporated as a powder into a host elastomermaterial to form a composite with a reduced, zero, or negative TCE. Italso teaches how the outside layer of the NTEs particles can be made tobe electrically conducting so that the elastomer composites can beelectrically conducting if the amount of NTEs filler exceeds thepercolation threshold. It also teaches how Land Grid Array (LGA)interposers can be advantageously fabricated using such composites. Italso teaches how the outside layer can be made of electricallyinsulating layer so that the elastomer composite is insulating. It alsoteaches how these NTEs devices can be used in pure or nearly pure formas solid foam by fusing or adhering the particles together.

A Negative Thermal Expansion system (NTEs) device for TCE compensationin elastomer composites and conductive elastomer interconnects inmicroelectronic packaging according to one aspect of the invention. A(NTEs) device comprising a first bilayer having an inner and an outerlayer, wherein the outer layer is of composed of a material having alower coefficient of thermal expansion than an inner layer of material,and a second bilayer having an inner and an outer layer, wherein theouter layer is composed of a material having a lower coefficient ofthermal expansion than a inner layer of material, wherein the first andsecond bilayers are joined together along a perimeter of the innerlayers of the material having a higher coefficient of thermal expansion.

Further, the first and second bilayers may be directly fused togetheralong the perimeter of the inner layers of material thereby forming ajoint and a remaining unjoined portion of the inner layers are able toseparate and flex thereby forming a cavity.

The NTEs device may also include an adhesion layer for joining the firstand second bilayer together, wherein the adhesion layer is an outer wallaround the cavity. In addition, the adhesion layer is either a fine lineof adhesive or a layer of material. Further, the adhesive layer may bechromium, titanium, or any materials that have adhesive properties tothe layers in question and that do not otherwise adversely affect thestructure.

The first and second bilayers of the device above may also be formed andconnected at a predetermined temperature rendering the device in a lowstress and geometrically flattened state, and the device becomesstressed at a temperature lower than the predetermined temperaturecausing a curvature in the first and second bilayers in opposingdirections that will increase the volume of the void between the firstand second bilayer and increase the overall volume occupied by thedevice. In addition, the predetermined temperature is an operatingtemperature of a final engineering application, e.g. a semiconductorchip having a predetermined temperature of approximately 100° C.

Further, the first and second bilayers may be joined by a circular bandwith right angle projections at the edge to contain the bilayers fromescaping their relative orientation relative to one another wherebyenhancing the negative coefficient of thermal expansion behavior.

According to another aspect of the present invention provides a methodfor fabricating a Negative Thermal Expanding system (NTEs) comprisingheating a substrate to a desired reference temperature, depositing ablanket of an organic release layer onto the substrate, depositing afirst layer of material onto the organic released layer, depositing asecond layer of material having a greater TCE value than the first layermaterial, depositing a decomposable polymer layer onto the second layerof material, patterning the decomposable polymer layer into disk shapeswith finger-like appendages radiating from the disk, depositing a thirdlayer of material having the same TCE value as the second layer ofmaterial over the decomposable polymer layer, depositing a fourth layerof material having the same TCE value as the first layer of materialonto the third layer material, depositing a layer of photoresist ontothe fourth layer material, lithographically patterning such that thedisk shapes of resist are left covering the decomposable polymer disksburied below and orienting the photoresist layer concentrically, etchingthrough an exposed area of all layers of material and the organicrelease layer the until the silicon substrate is encountered on thebottom, removing the photoresist layer, releasing a structure from thesubstrate, and annealing thermally to decompose the polymer core to formthe negative thermal expansion system device.

According to another aspect of the present invention provides a methodfor fabricating a NTEs devices comprising the steps coating a wafer witha thermally decomposable polymer, patterning the decomposable polymerinto repeating disk patterns, releasing the decomposable polymer fromthe wafer and forming a sheet of repeating patterned disks, suspendingthe sheet of released patterned decomposable polymer into a firstsolution with seeding compounds for electroless decomposition, removingthe sheet of released patterned decomposable polymer from the firstsolution, suspending the sheet of released patterned decomposablepolymer into a second solution to electrolessly deposit a first layermaterial onto both sides of the sheet, wherein the sheet of releasedpatterned decomposable polymer is held at a predetermined temperature,removing the sheet of released patterned decomposable polymer from thesecond solution, suspending the sheet of released patterned decomposablepolymer into a third solution to deposit a second layer of materialhaving a lower TCE value than the first layer of material onto bothsides of the sheet of released patterned decomposable polymer,separating the patterned disks from one another, and annealing thermallythe patterned disks to decompose the decomposable polymer and creating acavity in place of the decomposable polymer.

In addition, the patterned disks may be separated by ultrasonicagitation, wherein the disks break at the narrow point in the fingerbetween the disks exposing the decomposable polymer, and the polymerdecomposes during the annealing process thereby completing theseparation process. The patterned disks may also be separated by highshear mixing, wherein the disks break at the narrow point in the fingerbetween the disks exposing the decomposable polymer, and the polymerdecomposes during the annealing process thereby completing theseparation process. Further, the pattern disks may be separated bystretching the sheet of patterned disks in both the x and y directionsto crack the thin fingers between the disks and expose the decomposablepolymer at the narrow point of the finger, and submersing the sheet ofpatterned disks into a suitable solvent to dissolve the decomposablepolymer severs the ties between disks. Furthermore, the pattern disksmay be separated by stretching the sheet of patterned disks in thedirection 45 degrees to the x direction to crack the thin fingersbetween the disks and expose the decomposable polymer at the narrowpoint of the finger, and submersing the sheet of patterned disks into asuitable solvent to dissolve the decomposable polymer severs the tiesbetween disks.

According to another aspect of the present invention provides a CTEcompensated contact in a land grid array interposer comprising aninterposer with a plurality of contact holes, and a plurality ofcontacts in the plurality of contact holes, wherein the contacts areformed by placing the plurality of NTEs devices within a matrixelastomer and forming the matrix elastomer with the plurality of NTEsdevices into a desired shape.

According to another aspect of the present invention provides a (NTEs)device comprising a first layer of material, a second layer having agreater coefficient of thermal expansion (TCE) value than the firstlayer of material, a third layer of material having a TCE value the sameas the second layer of material, wherein the second and third layers arejoined together along a perimeter of the material, and a fourth layer ofmaterial having a TCE value the same as the first layer of material.

According to another aspect of the present invention provides aplurality of the NTEs devices may be joined together with a small amountof elastomeric adhesive directly instead of being incorporated into ahost medium. This would produce a foam-like solid with a negativecoefficient of thermal expansion.

According to another aspect of the present invention the NTEs devicereacts to temperature changes where the volume cavity increases as thetemperature of the device decreases below the predetermined temperatureand the volume decreases as the temperature of the device increases tothe predetermined temperature.

These and other embodiments, aspects, objects, features and advantagesof the present invention will be described or become apparent from thefollowing detailed description of the preferred embodiments, which is tobe read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts an exploded view of a conventional simple metal bilayer.

FIGS. 2 a-2 b depict a side isometric view of two simple metal bilayersjoined together back-to-back (symmetrically oriented to each other) atthe edges of the layers only to form a NTEs device, according to anembodiment of the present invention.

FIGS. 3 a and 3 b depict an exploded view of two simple fused bilayersjoined to each other at the edges only to form a NTEs device, accordingto another embodiment of the present invention.

FIGS. 3 c and 3 d illustrate a NTEs device at different temperatures,according to another embodiment of the present invention.

FIG. 4 depicts a cross-sectional view of a series of different stages ofthe fabrication of an NTEs device by one method, according to anotherembodiment of the present invention.

FIGS. 5 a-5 f depict cross-sectional views illustrating another methodof manufacturing a NTEs device, according to another embodiment of thepresent invention.

FIG. 6 depicts a cross-sectional view of a NTEs device, according toanother embodiment of the present invention.

FIGS. 7 a-7 b depict a top view of a plurality of NTEs devices,according to another embodiment of the present invention.

FIG. 7 c depicts a series of sequential top views illustrating anotherfabrication method for the NTEs devices, according to another embodimentof the present invention.

FIG. 7 d depicts both top and profile views of the last two stages offabrication by the method of 7 c-7 d.

FIGS. 8 a-8 b depict a series of sequential top views illustrating afabrication method for the NTEs devices, according to another embodimentof the present invention.

FIG. 9 illustrates a top view of a plurality of a NTEs device beingseparated, according to an embodiment of the present invention.

FIGS. 10-11 illustrate top views, cross-section views, and perspectiveviews of one type of completed NTEs device.

FIG. 12 a illustrates a portional cross-sectional view of a typical LGAinterposer.

FIG. 12 b is a portional 3D view of a composite material represented bya cubic cell boundary containing a plurality of NTEs randomly dispersedas a filler, according to another embodiment of the present invention.

FIG. 13 a illustrates a cross-sectional view of a high performance NTEsdevice with unconstrained bilayers, according to another embodiment ofthe present invention.

FIG. 13 b illustrates a top view of a high performance NTEs device withunconstrained bilayers, according to another embodiment of the presentinvention.

FIG. 14 a illustrates a top view of a freestanding NTEs device,according to another embodiment of the present invention.

FIG. 14 b depicts a series of sequential top views illustrating a3D-fabrication method for making the NTEs devices, according to anotherembodiment of the present invention.

FIG. 15 is a cross-sectional view of a solid foam made from a pluralityof NTEs devices, according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention now will be describedmore fully with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas being limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will convey the concept of the invention to those skilledin the art. In the drawings, the thickness of layers, regions, anddevices are exaggerated for clarity.

According to a preferred embodiment of the present invention, miniaturedevices comprised of dual opposed bilayers of material where the twobilayers are attached to one another at the peripheral edges only, andwhere the bilayers themselves are at minimum stress conditions at areference temperature defined by the temperature at which the bilayerswere formed. These devices have the unusual and technologically usefulproperty of volumetrically expanding upon lowering of temperature. Thesedevices are scalable from large (for example 25 micron diameter) to verysmall (hundreds of nanometers) depending upon the lithographic andfabrication resolution available.

FIG. 1 depicts an exploded view of a simple metal bilayer, according toan embodiment of the present invention. In FIG. 1, a metal layer 10 isfused to metal layer 11 having a lower TCE value than metal layer 10 ata predetermined temperature. The result is a simple metal bilayer at thepredetermined temperature 12 and a curled bilayer once it cools 13.

FIGS. 2 a-2 b depict a side isometric view of two simple bilayers joinedtogether back-to-back (symmetrically oriented to each other) at theedges of the layers to form a Negative Thermal Expanding system (NTEs)device, according to an embodiment of the present invention. The NTEsdevice shown in FIGS. 2 a and 2 b are structurally identical but areshown as they appear at different temperatures. FIGS. 2 a-2 b illustrateNTEs device in both the unstressed form, as shown in FIG. 2 a, and thestressed form, as shown in FIG. 2 b. The NTEs device is unstressed whenat a predetermined temperature, e.g. 100° C., and starts to stress asthe temperature of the device falls below the predetermined temperature.Referring to FIG. 2 a, a NTEs device 20 is shown having a first layer ofmaterial 21 connected to a second layer of material 22 having a higherTCE value than the first layer of material 21, an adhesion layer 23connected to the perimeter of the second layer of material 22 thusforming the sidewalls of a cavity 26, a third layer of material 24having the same TCE value as the second layer of material 22 isconnected to the second layer of material 22 by the adhesion layer 23,and a forth layer of material 25 having the same TCE value as the firstlayer of material 21 is connected to the third layer material 24. Theadhesion layer 23 may be of a variety of materials including a fine lineadhesive, e.g. chromium, or another metal, e.g. titanium. In addition,the second layer of material 22 may be directly fused to the outerperimeter of the third layer of material 24, both layers 22 and 24having the same TCE value, thereby forming a joint. Since the layers 22and 24 are only connected at the outer perimeter of each layer, theremaining unjoined portion of the layers 22 and 24 are able to separateand flex thereby forming a cavity.

FIGS. 3 a-3 d are perspective views of a saucer type Negative ThermalExpanding system (NTEs) device, according to another embodiment of thepresent invention. FIGS. 3 a-3 d illustrate a saucer type NTEs device inboth the unstressed form, as shown in FIGS. 3 a, 3 b, and 3 c, and thestressed form, as shown in FIG. 3 d. The NTEs device is unstressed whenat a predetermined temperature, e.g. 100° C., and starts to stress asthe temperature of the device falls below the predetermined temperature.Referring to FIGS. 3 a and 3 b, a NTEs device 30 is shown having a firstlayer material 31 connected to a second layer of material 32 having agreater TCE value than the first layer 31, an adhesion layer 33connected to the perimeter of the second layer of material 32 thusforming the sidewalls of a cavity 36, a third layer material 34 havingthe same TCE value as the second layer 32 is connected to the secondlayer of material 32 by the adhesion layer 33, and a fourth layer ofmaterial 35 having the same TCE value as the first layer 31 is connectedto the third layer of material 34. The Fabrication of NTEs device 30could be precisely carried out in a fashion illustrated in FIG. 4illustrating the sequential thin film deposition and etching steps.

FIG. 4 depicts a cross-sectional view of a series of different stages ofthe fabrication of a NTEs device by one method of fabrication, accordingto another embodiment of the present invention. In FIG. 4, a siliconwafer (not shown) or other suitable temporary substrate is heated to adesired predetermined temperature such as 100° C. The desiredpredetermined temperature is chosen to be about the operatingtemperature of the final engineering application, e.g., a chip operatingtemperature. Once the desired temperature is reached, a thin film 40,e.g. about 2 um, of a material is blanket deposited by evaporation (orother methods such as CVD, sputtering, etc.). Next, a second layer ofmaterial 41 having a greater TCE value than the first layer of material40 is deposited on top of the first layer of material 40, which has alower TCE value than the second layer 41. Then, a polymer layer 41consisting of a polymer type that will decompose or depolymerise atelevated anneal temperatures is deposited and patterned into diskshapes. One example of a polymer that is known to efficiently thermallydepolymerize at convenient temperatures is a polymethyl methacrylate,PMMA. Further, the polymer type may be, but not limited to,polyalphamethylstyrene, polyphenylene oxide, polyamide, polyimide,polyesters, polyurethanes, epoxies, photoresists of various types. Thus,using PMMA as an example, it would then be blanket coated on the secondlayer of material 41. Using either e-beam lithography to directlypattern it, or photolithography with photoresist steps or by othermeans, the PMMA is formed into disks with finger-like appendagesradiating from the disk, as shown in FIG. 4, item 46, which is a topview. Next, a third layer of material 43 having the same TCE value asthe second layer 41 is blanket deposited over the PMMA patterned disk 42and the exposed portions of the second layer of material 41. Then, aforth layer of material 44 having the same TCE value as the first layer40 is blanket deposited on the third layer of material 43. Next, on topof this structure, a thick photoresist 45, e.g. SU-8, is deposited ontop of the structure. The photoresist 45 is lithographically patternedsuch that disk shapes of resist are left covering the PMMA disks 43buried below and oriented concentrically to it. The photoresist 45should be of a slightly larger diameter than the PMMA disk 43 butslightly smaller than the radiating fingers so that a small part of thefinger projections are left un-covered. Then the wafer structure isetch, using reactive ion etching or ion milling, through the exposedareas of the four levels of metal 40, 41, 43, and 44 including theencased PMMA before the protected areas are affected, as shown in FIG.4, item 47. The photoresist 45 is then removed from the disk structures.The disk structures are then released from the wafer and annealedthermally to decompose the polymer core to form a negative thermalexpanding system (NTEs) device 48.

After the NTEs are formed by the above-described method, the NTEsdevices are then ready for use. One example of an application is theformation of a composite consisting of a multitude of these NTEs devicesmixed with the precursor components (or the melt) of an elastomeric orrubber material (e.g., a siloxane rubber). If the NTEs were mixed in asuitably large volume fraction, then the overall net TCE of theresulting rubber composite would be lower than it would be without theNTEs. Indeed, if enough of the NTEs were mixed in, the overall TCE couldbe rendered near zero or even into net negative values. Even a reductionin the composite TCE would offer significant technological advantage.

FIGS. 5 a-5 d depict various cross-sectional views of another negativethermal expanding system (NTEs) device along the manufacturing processin which the films are deposited sequentially and lithography defined,according to another embodiment of the present invention. Referring toFIG. 5 a, depicts a NTEs device on a silicon wafer 50. The NTEs deviceis composed of a lower organic layer 51 (which will disappear upon laterannealing), a first layer of material 52 deposited on the organic layer50, a second layer of material 53 having a greater TCE value then thefirst layer 52 is deposited on the first layer 52, a second organiclayer 54, e.g. DLC material or PMMA etc, deposited on the second layer53, a layer of Organic hard mask 55 is deposited on the organic layer 54wherein the organic layer 54 and the hardmask 55 are pattern into diskshapes as shown in FIG. 4, item 46, a third layer of material 56 havingthe same TCE value as the second layer 53 is deposited on the optionalorganic hard mask 55 and the exposed portion of the first high TCEmaterial 53, and a second layer of Low TCE material 57 is deposited onthe second layer of high TCE material 56.

In Reference to FIGS. 5 b and 5 c, depicts a cross-section view of anegative thermal expanding device during manufacturing of the device,according to one aspect of the present invention. As shown therein, asubstrate 50, a first Organic layer 51 deposited on the substrate, afirst layer of material 52 is deposited on the first organic layer 51, asecond layer of material 53 having a greater TCE value than the firstlayer 52 is deposited on the first layer of material 52, a secondorganic layer 54 is deposited on the second layer of material 53, anoptional layer of organic hard mask 55 is deposited on the second layerof organic layer 54, and resist layer 59 a is deposited on the organichard mask 55 and patterned to define the x-y size of the cavity. Next,the structure is introduced into an etching environment such as reactiveion etching (RIE) which etches the hard mask and the resist (59 a) untilthe exposed portions of the hardmask are etched away. Then etchingcontinues into the exposed organic layer and the resist (59 a) until theorganic layer is etched away. The resist layer (59 a) is now thinner butstill is present. It is then removed by another method (such as bysolvent or commercial resist stripper) that does not affect any otherportion of the structure. FIG. 5 c is what results, and it is now readyfor blanket deposition of layer 56 followed by 57.

In FIGS. 5 d-5 e, depicts the cross-sectional view of a NTEs deviceduring a portion of the process. As shown therein, after the x-y size ofthe cavity is defined, A third layer of material 56 having the same TCEvalue as the second layer 53 is deposited onto the cavity formingmaterials (the second organic layer 54 and the organic hard mask 55), alayer of low TCE material 57 is deposited on the layer of high TCEmaterial 56. Then a second layer of resist 59 b is deposited andpatterned onto the low TCE material 57. In FIG. 5 e, illustrates across-sectional view of a NTEs device after the all the layers haveetched away including organic layer 51 and the second resist layer hasbeen removed, according to another embodiment of the present invention.FIG. 5 f, illustrates a cross-sectional view of a completed NTEs device.Also implied, but not shown are fingers extending radially out as inshown in FIG. 4, item 46, which will provide pipe-like structure for therelease of gas formed in a subsequent organic decomposition step.

Referring to FIGS. 5 b-5 f, an exemplary method of manufacturing anegative thermal expanding device is as follows: An organic releaselayer 51 is blanket deposited onto a silicon wafer 50 or other suitabletemporary substrate and is heated to a desired reference temperaturesuch as 100° C. (This temperature is chosen to be close to the operatingtemperature of the final engineering application. e.g., a chip operatingtemperature). The organic layer 51 may be spin coated, or evaporated, orsprayed, or laminated, or coated by any other means onto the substrate50. The organic release layer may be either PMMA, alphamethyl styrene,polyphenyleneoxide, diamond-like-carbon (DLC), or any other likematerial. Next, a first layer of material 52, e.g. quartz (SiO₂), isblanket deposited on to the organic released layer 51. Note, an adhesionlayer may be needed for a particular material. The adhesion layer isvery thin so as not to significantly affect the mechanical operation ofthe device, e.g., an adhesive layer may be 200 angstroms of chromium ortitanium. The adhesion layer may be applied by vacuum evaporation,vacuum sputtering, spin coating or spray coating or any other suitablemethods. This would improve adhesion without introducing significantmechanical affects to the device because it is so thin. Similarlybetween any layers as needed. Next, a second layer of material 53, e.g.aluminum, having a greater TCE value than the first layer of material 52is deposited onto the first layer of material 52. Then, an organic layer54, e.g. DLC, consisting of a polymer type that will decompose ordepolymerise at elevated anneal temperatures is deposited and patternedinto disk shapes with finger-like appendages radiating from the disk, asshown in FIG. 4, item 46. Next, an optional organic hard mask 55 may bedeposited and patterned into disk shapes onto the organic layer 54.Next, a third layer of material 56 having a TCE value the same as thesecond layer of material 53 is blanket deposited over the DLC layer 54and Organic mask layer 55, if present. Then, a forth layer of material57 having the same TCE value as the first layer 52 is blanket depositedonto the material layer 57. On top of this structure then is deposited athick photoresist 59 b, such as SU-8. The photoresist islithographically patterned such that disk shapes of resist are leftcovering the organic disks (DLC, PMMA, SiLK, polyalphamethylstyrene,polyphenylene oxide, polyamide, polyesters, polyurethanes, epoxies,photoresists of various types, or any other like material) buried belowand oriented concentrically to it. It should be of a slightly largerdiameter than the organic disk but slightly smaller diameter than theprojecting fingers so that a small part of the finger projections areleft uncovered. Then, reactive ion etching, or ion milling the waferstructure will etch through the exposed areas of all levels of material51, 52, 54 (fingers only), 55 (fingers only), 56, and 57 before theprotected areas are affected. The SU-8 resist 59 b is then removed in away that does not etch or damage any remaining parts or materials. Thedisk structures are then released from the wafer by annealing thermallyto decompose the organic (PMMA, DLC, SiLK, polymer,polyalphamethylstyrene, polyphenylene oxide, polyamide, polyesters,polyurethanes, epoxies, photoresists of various types, or any other likematerial) core and the organic bottom layer 51 to form a micro expandingdevice 500. The released NTEs is shown in FIG. 5F sitting on but notadhered to the silicon substrate 50. It is shown in the stressed stateit would acquire when cooled to a temperature (for example, roomtemperature) below the processing temperature (for example, 100 deg.C.).

FIG. 6 is a cross-sectional view of a negative expanding device,according to another embodiment of the present invention. Referring toFIG. 6, the method steps of fabricating the NTEs device remainsessentially the same as explained above in reference to FIGS. 5 a-5 ewith added feature that vent holes (or vias) are formed in the secondlayer of high TCE material 56. This requires additional photolithographyand etching steps. More specifically, after the third layer of material56 is deposited, the layer is then patterned and vias are formed in thethird layer material 56. Thus, the vent holes are made perpendicular tothe plane of the wafer and offer a way to create the internal void (orcavity) by thermal decomposition just prior to depositing the final thinfilm layer, the forth layer of material 57. Thus, the fourth layer ofmaterial 57 closes these vent holes to provide an empty vacuum evacuatedchamber in the final NTES. Preferably, the second layer of high TCEmaterial 56 is of a thickness of about 1 to 2 um, and the sacrificialcavity material may be photosensitive polyimide (PSPI), DLC, PMMA, orany other suitable material, and the gap in the cavity may be largerthan the embodiment described above in reference to FIGS. 5 a-5 f (e.g.1000 nm compared to 100 nm). In addition, preferably, the pressure inthe cavity should be about 3 torr, but this is not necessary or evendesired. It is only typical of expected pressure when fabricated in thisway. Additionally, the organic layer 51 must be made of a differentmaterial than the material used in forming the cavity so that the devicewill not be released from the substrate during the process the cavity isbeing evacuated. For instance, the organic 54 should depolymerise at alower temperature than organic layer 51 so the two steps can be done atdifferent times. For example, the second organic layer 54 may be PMMAand organic layer 51 may be DLC.

FIGS. 7 a-7 c illustrate a top view of a plurality of negative expandingdevices in a repeating disk pattern separated by fingers which taper toa narrow point between the disks on a wafer. Referring to FIG. 7 a,depicts a top view of a wafer having a plurality of NTEs devices patternon the wafer. Referring to FIG. 7 a, illustrates a portional top view ofa plurality of NTEs devices on a wafer. In making these patterns, thewafer is coated with a thermally decomposable polymer, e.g. PMMA,alphamethypolystyrene, polyphenylene oxide, or some other thermallydecomposable polymer which are lithography definable. Preferably, thedisks are about 10 um in diameter, and the fingers separating the diskare about 2 um wide at the base of the finger and about 0.5 um at thenarrow portion of the finger.

FIG. 7 c depicts a series of sequential plan views illustrating afabrication method for making NTEs devices. A wafer is coated with athermally decomposable polymer like PMMA and patterned by e-beam orphotolithography (or by other means) into a repeating pattern of disksseparated by fingers which taper to a narrow point in between disks(Step 1). Once patterned in this way, the decomposable polymer sheet isreleased from the wafer and suspended into a liquid solution withseeding compounds for electroless deposition, e.g. coat with Pd seedcompound or sputter deposit monolayer of metal seed (Step 2). The sheetis then removed to another fluid (or solution), which is held at apredetermined temperature so the metal is plated at that temperature,e.g. 100° C., to electrolessly deposit a layer of metal having a greaterTCE value than the next layer of metal being deposited (or any otherhigh TCE material) Step 3. The sheet is then removed to another bath todeposit another layer of metal (or other low TCE material) having alower TCE value than the above layer of metal deposited in step 3 byeither electroless or electrodeposition (Step 4). Again, the platingbath should be held at the predetermined temperature. Alternatively, thepattern sheets could be sputter deposited on both sides with a firstmaterial followed by a sputter deposit on both sides with a secondmaterial having a lower TCE value than the first material. Still again,both depositions being done at the same predetermined temperature. Onceremoved, the disks are separated from one another by mechanical meanssuch as ultrasonic agitation or high sheer mixer (Step 5). The NTEswould break at the narrow weak point. Since broken or torn at thisnarrow point, the polymer layer inside is exposed in this smallcross-section, as shown in FIG. 7 d, items 71 and 72, so that during theannealing process the polymer decomposes thereby emptying the insidespace of the disks. The NTEs are then thermally annealed as shown inFIG. 7 d to decompose the internal polymer (Step 6), and the NTEs areformed having a hollow cavity in the center of the device, as shown inFIG. 7 d, items 71 a and 71 b. The resulting gasses from thedecomposition would be exhausted through the open vent fingers. Thevents are important to prevent rupture of the NTEs devices from excesspressure buildup during the annealing process. Even if rupture wereavoidable, the trapping of gasses could prevent efficient contraction ofthe NTEs device when desired.

FIGS. 8 a and 8 b are sequential plan view illustrating a fabricationmethod in much the same way as the previous case of FIGS. 7 a through 7d with a change in the method by which the metallized sheets aredisassembled into discrete disks. The steps 1-4 as stated above arerepeated. In this embodiment, the metallized sheets are mechanicallystretched in both the x and y directions to impart an elongation of thethin fingers between the disks, as shown in FIG. 8 a, or stretchmechanically in direction 45 degree orientation to the x (and y)direction only to elongate narrow vent tubes, and the decomposablepolymer is exposed at the narrow point of the finger, as shown in FIG. 9representing another embodiment of the present invention. The elongationand mechanical yielding takes place primarily or solely at the weaknarrow points. The weak narrow points of the fingers crack and exposethe decomposable polymer core. This is because the metal layers willcrack and not plastically deform like the polymer layers. Once thedecomposable polymer is exposed, then the sheet is submersed into asuitable solvent to dissolve the decomposable polymer and sever the tiesbetween disks. Alternatively, the solvent step may be bypassed and thesheet moves on to the annealing process which will burn out the polymerand simultaneously discretize the disks. If the solvent step is used,then in a subsequent step, the NTEs are thermally annealed todepolymerise the polymer core. Referring to FIG. 8 b, while some of thepolymer core will be dissolved away by the solvent, in all likelihoodthis will only penetrate a short distance into the vent tube. FIG. 8 bdepicts a plurality of NTEs device before and after the separating step.More specifically, FIG. 8 b illustrates the break in the narrow portionof the finger separating the NTEs devices before (80, 81, 82) and after(83, 81 a, 82 a) the thermal annealing process (step 6). In FIG. 8 bthere are two ways to perform step 6. One is with the use of a solvent,if the solvent step is used, then step 6 as shown is actually two steps,a solvent step and a thermal anneal step. The second way is if nosolvent step is used and only the thermal anneal step is used tocomplete step 6 as shown in FIG. 8 b. In addition, a cross-sectionalview of a NTEs device is shown anywhere except at the vent utters withthe decomposable polymer inside the cavity 81 and at the vent uttersshowing the elongation of the decomposable polymer 82 prior to theannealing process. After the annealing process is completed (step 6 togive 83), the cross-sectional view 81 a illustrates the hollow cavity ofthe NTEs device enclosed on all sides except at the vent, as shown initem 82 a. The thermal anneal process is still necessary to decomposethe bulk of the polymer core and form the cavity inside as illustratedin FIG. 8 b.

FIG. 9 illustrates an alternative method to separate the NTEs devices ona sheet, according to another embodiment of the present invention.Referring to FIG. 9, the sheet of NTEs devices is stretched mechanicallyin the direction 45 degrees to the x (and simultaneously they direction)only to elongate (in a single stretch) narrow vent tubes, and thedecomposable polymer is exposed at the narrow point of the finger. Oncethe decomposable polymer is exposed, the sheet is submersed into asuitable solvent to dissolve the decomposable polymer severs the tiesbetween disks. Alternatively, the solvent step may be bypassed and thesheet moves on to the annealing process which will burn out the polymerand discretize the disks.

FIG. 10 illustrates a top view and cross-sectional view of NTEs deviceafter all fabrication steps are completed. Referring to FIG. 10,illustrates the working of the NTEs device reacting to temperaturechanges. As shown in FIG. 10, the NTE's cavity increases in volume asthe temperature decreases below the predetermined processingtemperature, e.g. 100° C., and NTE's cavity decreases in volume as thedevice temperature increases. Since this is the reverse of how most purematerials behave, it has significant technological utility.

FIG. 11 is a perspective view of a NTEs device, according to anembodiment of the present invention. As shown in FIG. 11, the NTEs'cavity volume increases as the temperature of the device decreases, andthe NTEs' cavity volume decreases as the temperature of the deviceincreases.

FIG. 12 a is a cross-sectional view of a portion of a typical electricalinterposer. An interposer is a device which, for example, connects amultichip ceramic module to an organic printed wiring board. FIG. 12 aillustrates an interposer with carrier sheet 202 with contact buttons200 being held in place with metal bands 201. Figure is an explodedportional cross-sectional view of a contact, or conductive column, of aninterposer assembly, according to an embodiment of the presentinvention. More specifically, FIG. 12 b illustrates an application ofthe NTEs devices in a matrix elastomer, e.g. siloxane or rubber. Morespecifically, FIG. 12 b illustrates a composite material represented bya cubic cell boundary containing a plurality of NTEs devices randomlydispersed as a filler. In addition, FIG. 12 illustrates what happens tothe composite material upon cooling. For example, viewing at the scaleof an interposer such as shown in FIG. 12 b, where each contact is onthe order of a millimeter in size, the NTEs devices would be invisiblysmall to the naked eye. A substantial volume percent of the NTEs devicesor NTEs powder is mixed with an elastomer to form a contact 200. Inaddition, when the temperature is lowered the rubber will contractaccording to its coefficient of thermal expansion but the NTEs powderwill expand. Thereby, the overall resulting composite expansion orcontraction will be a function of the mix ratio and the coefficient ofthermal expansion values for both the elastomer and the negativeexpanding devices. Further, some mixes will give a net zero volumechange upon temperature change. Some mixes will result in net volumeexpansion upon cooling (very unusual and useful property), and somemixes will result in net contraction at a reduced extent than wouldhappen in the absence of the NTEs powder.

FIG. 13 a is a cross-sectional view of a NTEs device, according toanother embodiment of the present invention. FIG. 13 b is a top view ofa NTEs device, according to another embodiment of the present invention.Referring to FIG. 13 a, a NTEs is shown having unconstrained bilayers.The containment bands are made by a series of photolithography steps.For example, first the bottom of the band is formed, then in anotherstep the tall part of the band cross-section is formed, and then inanother step the top of the band is formed. A distinct design shown inFIGS. 13 a and 13 b offers larger values of negative thermal expansionby eliminating the bend strain imposed by the peripheral joining of thetwo bilayers in the other designs. This design utilizes an externalconstraint band made to keep the two free-floating bilayers properlyoriented to one another. This design can be fabricated with traditionalMEMs 2-D lithographic techniques, but it requires a greater number ofphotolithography steps.

In another embodiment, not shown in the figures, another type of highperformance NTE could be constructed by joining the two bilayerstogether at the edges with hinge type structures. This would allow freechanging of the angle between the two bilayers with little stress andyet keep them bound together. Hinge structures have been used in MEMsdevices such as mirrors that can flip up into position. Similar hingestrategies could be employed here to join the two bilayers.

FIG. 14 is a top view of a free standing NTEs device, according toanother embodiment of the present invention. FIG. 14 b illustrates aseries of sequential plan views of a 3-D bulk fabrication technique,according to another embodiment of the present invention. FIG. 14 b is asequential plan view illustrating a fabrication method much the same wayas the previous case of FIGS. 7 a-7 d with a change in the method bywhich the disks are patterned. The steps 2-6 as stated above, inreference to FIGS. 7 a-7 d, are the same. More specifically, FIG. 14 billustrates a method of 3-D bulk fabrication technique utilizing metalplating that can be varied by eliminating the sheet structure caused byhaving the disks connected by the tapering fingers. Instead, as shown inFIGS. 14 a and 14 b the lithographic pattern defines discrete disks butincludes a sacrificial end-cap structure on the end of each vent finger(step 1). This is to facilitate the tearing off at the weak narrow pointby mechanical agitation following all the metallization steps. Again,this tearing is necessary to expose the polymer core to the outsideenvironment in order to create an unblocked escape route for the gassesthat will be evolved during the polymer decomposition anneal step.

The number of these fingers does not have to be four. Only one vent perdisk is necessary to allow proper gas venting. The probability that atear will successfully occur goes up with duplication. But, only one isstrictly necessary. Further, the dimensions of the finger structureshown are for illustration only. For most purposes the length of thestructure between the disk and the narrow point should be as short aspractical in order to produce NTEs disks with minimal distortion from aperfect disk shape. The length and shape of the end-cap structure fromthe narrow point out to the end of the finger should be optimized totear off at the appropriate step in processing. They must endure theelectroless or electroplating steps intact, but then tear off uponapplication of the mechanical stressing by high sheer mixing or byultrasonic agitation. The hook structures are intended to allow theentanglement of disks in suspension so that the momentum of amechanically excited second disk attached to the first disk via the hookwill aid in tearing the weak point.

In another embodiment, not shown in the figures, a weak point in thedisk outer edge could be designed in rather than construct a vent fingerstructure. This way, when the disk were heated to decompose the organiccore, the pressure buildup would preferentially break through the weakpoint to provide a means of gas escape.

In another embodiment, one could mix the NTEs devices into a pasteinstead of into a thermosetting rubber compound. There are technologicalapplications of paste such as for thermal management in computerpackaging where maintaining good connection between the two opposingplanes of a heat source (e.g., a semiconductor chip) and a heat sink iscritical. Reducing, eliminating, or reversing the thermal expansionproperties of the paste could mitigate a very significant technologicalchallenge in high end computing.

FIG. 15 is a cross-sectional view of a plurality of NTEs devices forminga solid foam structure, according to another embodiment of the presentinvention. More specifically, FIG. 15 illustrates a plurality of NTEsdevices connected to one another with only a bare minimum of elastomerforming a solid foam structure. In addition, the minimum joiningconnections between the NTEs devices allows for a maximum degree offreedom in the expansion movement. One could also imagine joining themin other ways such as metallurgical bonding such that it would form atrue metal foam with negative expansion properties.

In another embodiment, any combination of materials can be used as longas the inside layer is of a higher TCE than the outside layer. Thiscould include for example metals, ceramics, polymers, and glasses.

In another embodiment, additional layers could be added to improveadhesion between layers, increase or decrease stresses as desired, or tochange the surface properties of the particle.

In another embodiment, a thin outer layer of metal is applied such thatit can impart electrical conductivity to the surface. The thin outerlayer can be made thin enough that it does not adversely affect theexpansion properties of the NTEs device. For example, 100 to 1000Angstroms of gold or silver.

In another embodiment, where electrical conductivity is not desired, anon-metallic thin coating can be applied as an insulator. This coatingcan be thin enough that it does not adversely affect the expansionproperties of the NTEs device.

The foregoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teaching can bereadily applied to other types of apparatuses and other scales ofdimension. The description of the present invention is intended to beillustrative, and not to limit the scope of the claims. Manyalternatives, modifications, and variations will be apparent to thoseskilled in the art.

1. A method for fabricating a plurality of negative thermal expandingsystem devices, comprising the steps of: coating a wafer with athermally decomposable polymer; patterning the decomposable polymer intorepeating disk patterns; releasing the decomposable polymer from thewafer and forming a sheet of repeating patterned disks; suspending thesheet of released patterned decomposable polymer into a first solutionwith seeding compounds for electroless decomposition; removing the sheetof released patterned decomposable polymer from the first solution;suspending the sheet of released patterned decomposable polymer into asecond solution to electrolessly deposit a first layer material ontoboth sides of the sheet, wherein the sheet of released patterneddecomposable polymer is held at a predetermined temperature; removingthe sheet of released patterned decomposable polymer from the secondsolution; suspending the sheet of released patterned decomposablepolymer into a third solution to deposit a second layer of materialhaving a lower TCE value than the first layer of material onto bothsides of the sheet of released patterned decomposable polymer;separating the patterned disks from one another; and annealing thermallythe patterned disks to decompose the decomposable polymer and creating acavity in place of the decomposable polymer.
 2. The method of claim 1,wherein separating the patterned disks comprises the following step:separating the patterned disks by ultrasonic agitation, wherein thedisks break at the narrow point in the finger between the disks exposingthe decomposable polymer, and the polymer decomposes during theannealing process thereby completing the separation process.
 3. Themethod of claim 1, wherein separating the pattern disks comprises thefollowing step: separating the patterned disks by high shear mixing,wherein the disks break at the narrow point in the finger between thedisks exposing the decomposable polymer, and the polymer decomposesduring the annealing process thereby completing the separation process.4. The method of claim 1, wherein separating the pattern disks comprisesthe following steps: stretching the sheet of patterned disks in both thex and y directions to crack the thin fingers between the disks andexpose the decomposable polymer at the narrow point of the finger; andsubmersing the sheet of patterned disks into a suitable solvent todissolve the decomposable polymer severs the ties between disks.
 5. Themethod of claim 1, wherein separating the pattern disks comprises thefollowing steps: stretching the sheet of patterned disks in the xdirection (allowing the stretching of both x fingers and y fingers in asingle step) to crack the thin fingers between the disks and expose thedecomposable polymer at the narrow point of the finger; and submersingthe sheet of patterned disks into a suitable solvent to dissolve thedecomposable polymer severs the ties between disks.
 6. The method ofclaim 1, wherein patterning the decomposable polymer into repeating diskpatterns further comprises the disk patterns being separated by fingerswhich taper to a narrow point in between the disks.